Sulfur dioxide is a constituent of many waste gases, such as, for example, smelter gases, flue gases, off gases from chemical manufacturing processes, ore roaster gases, and stack gases from coal- and oil-burning furnaces and boilers. Contamination of the atmosphere by sulfur dioxide, whether present in dilute concentrations of 0.05 to 0.3 volume percent as in power plant flue gases or in higher amounts of 5 to 10 percent as in ore roaster gases, has been a public health problem for many years due to its irritating effect on the respiratory system, its adverse affect on plant life, and its corrosive attack on many metals, fabrics and building materials. Millions of tons of sulfur dioxide are emitted into the atmosphere each year in the United States due to combustion of fuel oil and coal; a major amount of such sulfur dioxide being produced in the generation of electric power.
Since the reduction of the sulfur dioxide content of stack gases is the key to the production of useful energy from our abundant fuels (coal and high sulfur oil) is an environmentally acceptable manner, many methods have been proposed, and are presently under study for the removal of sulfur dioxide from such gases. It is estimated that there are close to 50 sulfur dioxide removal processes presently under investigation in the United States. While the processes appear technically feasible, the expense of the sulfur dioxide removal is substantial. Some of the more common processes involve scrubbing of the stack gas and precipitation of the sulfur dioxide with limestone as calcium sulfite or, following oxidation, as calcium sulfate. Scrubbing of the very large effluent gas quantities, as well as collection and disposal of the solid precipitate from the scrubbing liquid, are expensive.
An inherently less expensive method for removing the sulfur dioxide is based on the catalytic reduction thereof with carbon monoxide or some other reductant. Neither scrubbing of a gas by a liquid nor the separation of a solid from a liquid are required in this method. This method has tried with many different catalysts but, to date, to the best of Applicant's knowledge, such methods have one or more of three major difficulties. Initially, burners, such as those operated by electrical power generation, run on fuel mixes with excess air or "lean fuel mixes". This is done to prevent the formation of explosive carbon dust and to derive more energy from the fuel. As a result of the use of the lean fuel mix, the stack gas is rich in oxygen. This oxygen poisoned many of the catalysts tried in the past, thus killing the catalytic activity thereof and reducing the overall effectiveness of the reduction process. Secondly, certain of the catalysts utilized catalyzed the reduction of water by carbon monoxide to form carbon dioxide and hydrogen, or catalyzed the reaction of water and sulfur to hydrogen sulfide and oxygen. Hydrogen reacts with sulfur to form hydrogen sulfide at temperatures as low as about 200.degree. C, and, thusly, in either case, the sulfur dioxide is converted into another toxic material. Thirdly, certain of the non-specific catalysts catalyzed the oxidation of carbon monoxide by sulfur to form carbonyl sulfide, another highly toxic gas. These difficulties arise because of the non-specific nature of the catalytic material.